This invention is related to an earlier invention of the same inventors entitled "Multiple Feed RF Coils" and assigned to the assignee of this invention. The prior invention was filed in the U.S. on Nov. 7, 1984 and received Ser. No. 668,944, and which issued as U.S. Pat. No. 4,613,837 on Sept. 23, 1986.
In MR data acquisition systems, radio frequency (RF) coils are used to transmit RF signals which nutate nuclei that have been aligned by static magnetic fields. After the RF signals are removed, the nutated nuclei subsequently return to their former aligned positions. During this return, free induction decay (FID) signals are generated. These signals are detected by RF coils and provide data used to generate display images, for example. The most commonly nutated element in magnetic resonance imaging (MRI) is hydrogen. Other elements are also nutated, such as, for example, sodium. Also for a long time it has been known that phosphate can provide unique information. Thus, phosphate has been nutated to obtain MR spectroscopic data. For example, it is known that there is an abnormal abundance of phosphate in tissue that is tumorous. Accordingly, during some MRI procedures it is useful to obtain MR data based on the nutation of phosphor in the patient.
Surface coils are special RF probes that are designed to operate when juxtaposed to a portion of the patient. The proximity improves the effect of the RF signal. For example, surface coils are used to obtain images of a female patient's breast using the nutation of hydrogen nuclei. By using another frequency, the phosphor nuclei are nutated and spectroscopic data is obtained showing whether there is an abnormal accumulation of phosphor in the breast. If such an accumulation is found it is a reliable, early indication that a cancerous condition exists. This indication is obtainable long before other known tests for cancer can provide reliable data. The early detection of cancer is necessary for effecting a cure. Thus, it is extremely important to obtain such information as soon as possible.
In the past, it has been the practice to first obtain the MRI data using one RF surface coil or probe tuned for the Larmor frequency of hydrogen and subsequently obtaining phosphor spectrometric data using another RF probe tuned for the Larmor frequency of phosphor. The different coils were used since different frequencies are necessary to obtain the nutation of the different nuclei. It has not been common to use the same coil for the different frequencies because, among other things, of the difficulties in matching the impedance of the same probe to the characteristic impedances of the generator and receiver at two different frequencies.
However, the use of the same coil has indeed been suggested for radio frequency probes operating at different frequencies in nuclear magnetic resonance. For example, the U.S. Pat. No. 4,446,431 teaches a single coil for use at two frequencies. The tuning is accomplished remotely by using a transmission line wherein the high frequency signal is coupled at one point and the low frequency signal is coupled at another point on the transmission line.
The junction point for the low frequency signal in the single sample coil of the patent is extremely critical, as is the transmission line length. Another drawback of the double tuned single coil probe of the noted patent is that it has to be separately tuned for each sample being nutated. When the probe is used for acquiring data of inanimate objects this presents more of an inconvenience than a problem. The inconvenience is largely due to the increase in throughput time. However, when the sample is a human being then the retuning for each patient is not only time consuming for the clinic which decreases throughput, but is also time consuming for the patient and detrimental to the patient's comfort.
The prior art also teaches the use of wide band probe arrangements for MR spectrometers. See for example U.S. Pat. Nos. 4,075,552 and 4,129,822. Both patents teach tunable tank circuits that are individually tuned to determine the MR frequencies of samples. The coil, and associated circuity are designed to match the impedances of both the transmitter and receiver at a wide range of frequencies. The '552 patent uses an auto-transformer as the inductance of the tank circuit while the '822 patent uses a plurality of directly connected coils, in a series parallel arrangement with a switching arrangements for inserting and removing a coil for different frequency ranges.
The noted prior art, in effect, teaches single coils or directly coupled coils with the resonant frequency being varied by variable capacitors, inductors or combinations thereof. Hence with the prior art devices it is necessary to tune the circuit each time the frequency is changed from high to low or vice-versa and/or either single or unbalanced coils are used in the RF probes. With probes made with unbalanced coils it is necessary to tune the probe for each patient. Periodic tuning operations are not sufficient to set the tuning for a plurality of specimens of samples (patients).
Accordingly, there is a need for surface probes for use in MR operations that can be used at a plurality of frequencies without requiring separate tuning for each of the frequencies and wherein the tuning is not a function of the impedance of the sample being probed. Therefore, it is an object of the present invention to provide radio frequency balanced probes for magnetic resonance operations which are tuned to a plurality of frequencies so that the probe can be used for acquiring MR data of different elements (hydrogen, sodium and/or phosphor nuclei, for example) without the necessity of removing and/or retuning the probes between the uses of the different frequencies. Ideally the RF probes provided also will not require tuning when used on different patients.
A balanced probe is defined herein as one in which "virtual" short circuit planes are produced by means of excitation of equal amplitudes and opposite polarities at an even number of feed points symmetrically spaced apart from each other. The balanced probes have equal potentials at opposite points on the circuits and thus the opposite points of the circuit can be considered interconnected or short circuited.